The human body imposes stringent requirements on electrical conductors that are implanted in it. Cardiac pacing and defibrillation leads are subjected to flexure with every heartbeat which total approximately 100,000 per day or over 30 million a year. Conductors and their insulators in parts of the leads that are remote from the heart often undergo stresses of various kinds during body movement. A living body also constitutes a site that is chemically and biologically hostile to anything that invades it. Therefore, it is not surprising that with prior art leads, the conductors and insulation deteriorated in a period that was shorter than desired for a device that requires major surgery to correct.
With the advancement of implantable defibrillator and pacemaker technology, more electrodes, and consequently more conductors are used to provide sensing, pacing, defibrillation, and other functions Because of the increase in number of conductors, it is important that their individual size does not increase, and preferably, that it decrease. Defibrillator conductors in particular are required to carry much larger currents than other types of conductors, and therefore must have very low resistances over their lengths. For example, a defibrillator conductor may carry 35 amperes or more of current, whereas a pacing lead may carry only about 100 milliamperes. Because defibrillators are usually implanted in the abdominal region rather than in the typical pectoral pacemaker implant site, defibrillator leads must usually be longer than pacemaker leads. The defibrillator conductor must be about 4 ohms or less over a length of up to about 110 centimeters; the pacemaker conductor may be closer to 40 ohms, and as much as about 200 ohms, over about 60 centimeters. Therefore, a small diameter, low resistance conductor with a high fatigue life in the body is desired.
Unfortunately, the materials with the highest conductivities, such as copper, silver, and gold, also tend to have low yield strength, and consequently low fatigue life. Also unfortunate is the fact that for coil construction, the variables that increase conductivity also decrease fatigue life; that is, conductivity increases for increased wire diameter, increased pitch, and decreased mean coil diameter, whereas fatigue life increases for decreased wire diameter, decreased pitch, and increased mean coil diameter.
In U.S. Pat. No. 4,640,983, which is incorporated herein by reference, Comte describes a conductor device comprising at least one spiral formed from a plurality of electrical conductors arranged to form a single-layer winding, wherein each of the conductors is formed from several wires bundled in a ropelike configuration, and wherein each wire consists of the same material over its entire cross section. Comte attempts to solve tho problem of combining strength and conductivity by combining different material wires in a conductor thus having some wires of each of two materials, one material being strong arid the other one being highly conductive. As described, if the sheathing which normally protects the wires is damaged, the wires of both materials would come in contact with blood or other cells and tissues of the patient's body. Silver and copper are the most likely candidates for the highly conductive material; however, they are not biocompatible and will readily corrode in body fluids.
Drawn brazed stranded (DBS) material has been used in the past to make strong wire having low electrical resistance for use in cable or coil. This material is formed from six segments of a strong material such as 316L stainless steel or MP35N cobalt-nickel-chromium-molybdenum alloy which enclose a core of silver; the segments and core are brazed together with silver securely connecting all of the segments, then drawn down to the final wire diameter. This wire has a tendency to kink and is difficult to wind into a uniform helical coil. Nonuniformities, and especially kinks, render the wire prone to fatigue. Also, because silver is used to braze the materials together, silver remains exposed on the outside of the wire. Because silver has been implicated in the degradation of polyurethane materials, which are often used as insulators in implantable products, the choice of insulating materials is limited if DBS is used. Furthermore, any breach in an alternative insulating material such as silicone rubber would expose the silver to body fluids, subjecting it to corrosion. Because silver extends from the wire exterior into its interior, corrosion of silver in brazed joints could lead to complete failure of the wire.